1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing system, and particularly, to an orthogonal frequency division multiplexing system with PN-sequence.
2. Description of the Related Art
A large number of references related to the orthogonal frequency division multiplexing (OFDM) synchronization algorithm exist, and can be divided into the time domain and the frequency domain according to the types of solution, i.e. before and after the Inverse Fast Fourier Transform (IFFT), or into the Non-Data-Aided (also known as blind estimation) and Data-Aided according to data mode, both of which usually use training symbols or cyclic prefixes, pilot symbols and so on.
The concept of using the cyclic prefix is mainly that every OFDM symbol has periodicity after being added with a cyclic prefix, so we only need to use a correlator to estimate the correlation of the cyclic prefixes in the OFDM symbols, and through the Maximum Likelihood Estimation Algorithm, easily calculate the starting point of each OFDM symbol and the offset of carrier frequency as described in the prior related art reference (1) J. J. van de Beek, M. Sandel, and P. O. Borjesson, “ML estimation of timing and frequency offset in OFDM systems,” IEEE Trans. Signal Processings, vol. 45, pp. 1800-1805, July 1997 and U.S. Pat. No. 5,889,759.
The concept of using the training symbol to perform synchronization is that the two PN-sequences in the frequency domain may become periodic time domain signals after passing the IFFT at a transmitting terminal, and the synchronization of timing offset can be accomplished by acquiring the timing metric for estimating time through the operation of the correlation and calculating the maximum of the timing metric though the Maximum Likelihood Estimation Algorithm. In addition, the concept of using the timing metric to estimate the frequency offset is mainly that the first training symbol is the equal data of N/2 length in the timing domain, and thus the frequency offset can make the two equal data have a phase shift expressed by φ=π{circumflex over (ε)}, in which {circumflex over (ε)} is the frequency offset to be estimated. Therefore, the estimation of frequency offset can be accomplished by just calculating the angle of timing metric and dividing the angle by π as described in the prior related art references (2) Schmidl, T. M. and Cox, D. C., “Robust frequency and timing synchronization for OFDM”, IEEE Transactions on Communications, vol. 45, pp. 1613-1621, December 1977.; (3) P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction,” IEEE Trans. Commun., vol. 42, pp. 2908-2914, October 1994 and Republic of China Patent Nos. 567689 and No. 560148.
In addition, in the prior related art reference (4) Landstrom, D., Wilson, S. K., van de Beek, J.-J., Odling, P., Borjesson, P. O., “Symbol time offset estimation in coherent OFDM systems,” IEEE Transactions on Communications, vol. 50, pp. 545-549, April 2002, the methods of the pilot symbol and the cyclic prefix are combined. Though the synchronization method just based on the cyclic prefix can be improved, the position of the pilot symbol must be arranged properly, thereby achieving a better performance.
The methods mentioned above are all data-aided methods. Though the method is of less complexity, due to the addition of extra data, the bandwidth utilization is lowered consequently, which is the biggest disadvantage of using the conventional data-aided method.
Therefore, it is necessary to provide an orthogonal frequency division multiplexing system to solve the above problems.